In conventional slagging furances for steam generation and the like, solid carbonaceous fuels, such as pulverized bituminous or semibituminous coal, are fed into a reaction zone. The combustion temperature is maintained at or above the ash fusion temperature in order to convert a major portion of the non-combustible ash present in the fuel to molten slag. Such furnaces have been designed for atmospheric or near atmospheric operating pressure and, as a result, have generally been relatively very large, have required the use of ceramic structures to prevent erosion of the combustion chamber by the high temperature combustion products, have released large quantities of pollutants into the atmosphere, and have been severely limited as to the applications for which they could be utilized. The relatively large size of such furances has resulted in relatively high heat losses, lower overall thermal efficiency, and undesirably low power density, that is, thermal power output per unit of volume of the furnace.